1. Field of the Invention
The present invention relates to a delay adjustment unit and method, an optical network unit, and a communication system. More particularly, the present invention relates to a delay adjustment unit and method which perform delay adjustments when transmitting data to a network. The invention further relates to an optical network unit coupled to an optical access network system, which transmits upstream data after making a delay adjustment thereto. The present invention also relates to a communication system which transports a data signal after making delay adjustments.
2. Description of the Related Art
Increasing numbers of telecommunication and multimedia services are provided today to serve for the growing market needs, including video on demand, cable TV, and high-speed access to computer networks. To provide those high-bandwidth services without raising the cost to subscribers, optical access network systems are expected to play an essential role, connecting subscriber premises to the nearest local office exchange through fiber optic cables, rather than conventional metallic wires.
One example of such optical access network systems is called xe2x80x9cPassive Double Star (PDS),xe2x80x9d which enables a plurality of subscribers to share a single optical fiber line by using star couplers. Particularly in Europe, the Passive Optical Network (PON) system, synonymous with PDS, is of great interest as an enabling technology for the xe2x80x9cFiber to the Homexe2x80x9d (FTTH) services. In the scenarios toward FTTH, the access network has to provide guaranteed bandwidths and quality of services to meet the requirements for real-time voice and video communication. To this end, the Full Service Access Networks (FSAN) initiative has a central role in the development of ATMPON systems based on the Asynchronous Transfer Mode (ATM) technologies. The FSAN is an organization formed by major telephone companies to promote worldwide optical network businesses.
FIG. 25 shows a typical structure of an ATM-PON system. In this system, optical network units (ONUs) 101a to 101n are deployed in subscriber premises 100a to 100n, while an optical line terminal (OLT) 201 is placed in a local office 200. Fiber optic cables and a star coupler 300 permit those ONUs 101a to 101n to connect to the OLT 201 in a point-to-multipoint fashion. Within the subscriber premises 100a to 100n, telephone equipment and/or CATV equipment is coupled to the ONUs 101a to 101n. Connected to the OLT 201 in the local office 200 is ATM and ISDN switching equipment 202.
In the downstream direction, the local office 200 broadcasts data (i.e., downstream cells) toward the subscriber premises 100a to 100n over a single optical fiber cable. The star coupler 300 splits the optical signal into a plurality of signals in a tree and branch form, so as to deliver the information to individual subscribers"" ONUs. In the upstream direction, ATM cells are transmitted from the subscriber premises 100a to 100ntoward the local office 200 over the same branch cables. The star coupler 300 consolidates them into a single optical signal for delivery to the local office 200 over a single fiber cable.
As explained above, the ATM-PON system is defined as an ATM-based, optically-coupled access network which provides point-to-multipoint (1:n) connections between a local office and a plurality of customers through the use of a star coupler 300. The OLT 201 communicates with the ONUs 101a to 101n, sending cells back and forth over the ATM-PON network. The ONUs 101a to 101n transmit upstream cells (e.g., cells xe2x80x9ca,xe2x80x9d xe2x80x9cb,xe2x80x9d . . . xe2x80x9cnxe2x80x9d in FIG. 25) with different time delays to prevent the cells from colliding with each other. Actually, the OLT 201 controls those delays by supplying the individual ONUs 101a to 101n with parameters called the xe2x80x9cequalization delaysxe2x80x9d in some downstream cells. Because the amount of delay time may vary with the operating conditions (e.g., variations in the ambient temperature), the OLT 201 always monitors the deviation in upstream cell delays, and if necessary, updates the equalization delay parameters to compensate for the observed deviation.
Such a conventional system has a problem in handing new equalization delay parameters. Suppose, for example, that a certain ONU (e.g., ONU 101a) has received a new equalization delay parameter that is smaller than the current one. The ONU 101a activates this new parameter. But if this is done in the middle of the transmission of upstream cells, some of those cells would collide with each other, resulting in a loss of transmission data.
Normally, the ONUs 101a to 101n has two separate units to process transmission data: one for processing downstream cells, and the other for processing upstream cells. To handle downstream cells, the downstream cell processing unit uses a clock extracted from the downstream signal. On the other hand, the upstream cell processing unit handles upstream cells, using a clock signal that is supplied by its integral oscillator (PLO). Note that this PLO clock signal is phase-locked with the above downstream clock signal.
When a new equalization delay and other related information is received through the downstream channel, the ONU has to make this new setup information reflected correctly in the downstream cell processing unit. To achieve this, the two cell processing units have to operate in phase with each other. In actuality, however, tracking and maintaining a correct clock phase is not an easy task. Particularly, bursty errors in a downstream cell could cause a large amount of phase shift, which cannot be dealt with the receiving ONU""s PLL alone. Because of the lack of continuous downstream data, conventional ONUs are unable to extract clock timing from the downstream signal. When there is a large phase difference between the upstream clock and downstream clock, conventional ONUs forcibly adjust their clock phases in an attempt to cancel the phase difference at a time. This hasty adjustment causes some data loss, meaning that conventional ONUs cannot guarantee correct data delivery in such a situation.
Taking the above into consideration, an object of the present invention is to provide a delay adjustment unit which efficiently adjusts the delay of cells to make high quality data transmission possible.
To accomplish the above object, according to the present invention, there is provided a delay adjustment unit which transmits upstream data composed of a plurality of data blocks making a delay adjustment to them. This delay adjustment unit comprises the following elements: a downstream receiver which receives control information on which data blocks are granted or not granted, together with an equalization delay update request that requests the delay adjustment unit to update an equalization delay parameter being held therein; a delay adjustment controller which controls the delay time of the granted or non-granted data blocks specified in the control information, when the equalization delay update request is detected; and an upstream transmitter which transmits the data blocks with the adjusted delay times.
Another object of the present invention is to provide an optical network unit which efficiently adjusts the delay of cells to make high quality data transmission possible.
To accomplish the above second object, according to the present invention, there is provided an optical network unit, coupled to an optical access network system, which transmits upstream data composed of a plurality of upstream cells after making a delay adjustment to the upstream cells. This optical network unit comprises the following elements: a downstream receiver which receives control information on which upstream cells are granted or not granted, together with an equalization delay update request that requests the optical network unit to update an equalization delay parameter being held therein; a delay adjustment controller which controls the delay time of the granted or non-granted data blocks specified in the control information, when the equalization delay update request is detected; and an upstream transmission unit which transmits the upstream cells with the adjusted delay times.
Still another object of the present invention is to provide a communication system which efficiently adjusts the delay of cells to make high quality data transmission possible.
To accomplish the above third object, according to the present invention, there is provided a communication system which transports a data signal after making delay adjustments. This system comprises an optical line terminal and an optical network unit. The optical line terminal comprises the following elements: a downstream data producing unit which produces downstream data containing control information on which upstream cells are granted or not granted; and a downstream transmitter which transmits the produced downstream data. The optical network unit comprises the following elements: a downstream receiver which receives the downstream data; a delay adjustment controller which controls delay times of the granted or non-granted upstream cells specified in the control information, when an equalization delay update request is detected; and an upstream transmitter which transmits the upstream cells with the adjusted delay times.
Still another object of the present invention is to provide a delay adjustment method which efficiently adjusts the delay of cells to make high quality data transmission possible.
To accomplish the above fourth object, according to the present invention, there is provided a delay adjustment method which transmits upstream data composed of a plurality of upstream cells after making a delay adjustment thereto. This method comprises the steps of: receiving downstream data containing control information on whether the transmission of each upstream cell is granted or not granted; when an equalization delay update request that requests an update of an equalization delay parameter is received, controlling delay times of the granted or non-granted upstream cells, according to the received control information; and sending upstream data with the adjusted delay times.